The present invention relates to calibrating electrically actuated hydraulic valves of the type that control the application of hydraulic fluid to hydraulic cylinders in various machines. More specifically, the present invention relates to a method of calibrating valves associated with hydraulic cylinders in asphalt screeds in earth moving machine, and in other similar machines. The invention contemplates using sensors, such as acoustic or ultrasonic sensors that are relatively noisy, to monitor cylinder movement during the calibration process.
Various types of machines, such as earth grading and paving machines, operate in an automatic or semi-automatic mode using one or more sensors, such as ultrasonic or acoustic sensors, to control the movement of machine elements. For example, an asphalt paving machine deposits a layer of asphalt as it moves along a roadbed, with the thickness of the layer being based on the relative height of an adjacent reference surface or an adjacent reference string. It is common to measure this relative height with an ultrasonic transducer that directs pulses of sonic energy downward, and measures the time required for the sonic energy to reach the reference surface or string and to be reflected back to the transducer. This measured time is directly related to the distance from the transducer to the reference surface or string.
An asphalt paving machine typically uses a screed which is pulled behind the machine to control the thickness of the asphalt layer. The screed is pulled by a pair of screed tow arms which extend forward along the sides of the paving machine. Raising the tow points of the screed tow arms causes the angle of attack of the screed to change, resulting in a thicker layer of asphalt. Lowering the tow points of the screed tow arms correspondingly reduces the thickness of the asphalt layer. The tow points of the screed tow arms are raised and lowered by a pair of hydraulic cylinders that receive hydraulic fluid through electrically actuated hydraulic valves. The valves receive command signals which vary in level from a control circuit that is responsive to an ultrasonic transducer. The ultrasonic transducer may be mounted on one of the tow arms.
When the ultrasonic control system is added to the paving machine, it is necessary to calibrate various aspects of the control system, including the hydraulic valves that raise and lower the tow points. In the past, the valves in paving machines having ultrasonic sensors have been calibrated manually, or calibrated using sensors other than the ultrasonic sensors, since it has been found that ultrasonic sensors typically are too noisy to be used in automatic calibration techniques for valves. Manual calibration is time consuming and providing other sensors for valve calibration adds to the cost and complexity of the systems. It is seen, therefore, that there is a need for an automated method for calibrating valves in a paving machine control system in which the calibration method utilizes the ultrasonic transducer that is an integral part of the control system.